A manufacturing method of an abrasive polycrystalline sintered ceramic grain for grinding, using high-density alumina (Al.sub.2 O.sub.3) as a base material and a sol/gel manufacturing process, is well-known. Patent application JPA 56-32369 (corresponding to U.S. Pat. No. 4,518,397 and EP-A-24099) describes that an alumina hydrate may be gelled together with a precursor comprising at least one kind of reforming component, then dried and sintered. The reforming component used in this case includes oxides of Co, Hf, Mg, Ni, Zn and Zr. Patent application JP-A-60-231462 (corresponding to U.S. Pat. No. 4,623,364 and EP-A-152768) describes a sol/gel method for accelerating the manufacture of high-density alumina by adding alpha-alumina seed crystals, which may be added to the sol as a crystal growth control agent comprising oxides of Si, Cr, Mg and Zr.
Patent application JP-A-61-254685 (corresponding to U.S. Pat. No. 4,744,802 and EP-A-200487) states a method for adding alpha-alumina, alpha-ferric oxide, or their precursors into a sol as a nucleating agent, and includes a statement that the gel contains precursors of oxides of Mg, Zn, Co, Ni, Zr, Hf, Cr and Ti.
Although producing a sintered alumina abrasive grain by a sol/gel process yields strong fine abrasive polycrystal grains, the crystal size of the grain is coarse and does not have uniform size without addition. Therefore, the grain is improved by adding reforming components, such as MgO and ZrO.sub.2, into the alumina sol material, as described above. However, because low-hardness substances like spinel, are formed in the grain boundary of the polycrystalline abrasive grain, the average hardness and strength of the abrasive grain could be improved even further. Although the hardness of the grain has been addressed to some extent by these methods, further improvements are both possible and desirable.